Calcium-dependent K current in plasma membranes of dermal cells of developing bean cotyledons

Abstract

In developing seeds of bean (Phaseolus vulgaris L.), phloem-imported assimilates (largely sucrose and potassium) are released from coats to seed apoplasm and subsequently retrieved by the dermal cell complexes of cotyledons. To investigate the mechanisms of K⁺ uptake by the cotyledons, protoplasts of dermal cell complexes were isolated and whole-cell currents across their plasma membranes were measured with the patch-clamp technique. A weakly rectified cation current displaying a voltage-dependent blockade by external Ca²⁺ and acidic pH, dominated the conductance of the protoplasts. The P haseolus v ulgaris Cotyledon Dermal-cell pH and Calcium-dependent Cation Conductance (Pv-CD-pHCaCC) was highly selective for K⁺ over Ca²⁺ and Cl⁻. For K⁺ current through Pv-CD-pHCaCC a sigmoid shaped current–voltage (I–V) curve was observed with negative conductance at voltages between −200 and −140 mV. This negative K⁺ conductance was Ca²⁺ dependent. With other univalent cations (Na⁺, Rb⁺, NH₄⁺) the currents were smaller and were not Ca²⁺ dependent. Reversal potentials remained constant when external K⁺ was substituted with these cations, suggesting that Pv-CD-pHCaCC channels were non-selective. The Pv-CD-pHCaCC would provide a pathway for K⁺ and other univalent cation influx into developing cotyledons. These cation influxes could be co-ordinated with sucrose influx via pH and Ca²⁺ dependence.